In certain types of spacecraft, such as geosynchronous orbiting (GEO) spacecraft that comprise large deployed reflectors and/or antennas, pitch gravity-gradient torques may result from the geometric arrangement of the various components of the spacecraft. For example, in the case of a mobile user objective system (MUOS), a spacecraft may comprise a solar cell array and a deployed mesh reflector or antenna, with a separation between the solar cell array and mesh reflector/antenna being on the order of 50 feet. This separation between spacecraft components results in a difference in gravitational force with the orbited body (spacecraft) in accordance with the well-known inverse-square gravitational law, and produces a resultant pitch gravity-gradient disturbance torque. In addition, spacecraft may be subject to a number of other types of environmental disturbance torques.
Referring to FIG. 1, schematically shown therein is an illustrative, but not limitative, example of an asymmetrically arranged spacecraft 100 of the type contemplated above, comprising a main body 101 including power supply systems, sensors, thrusters, fuel storage means, etc. A plurality of deployed solar cell arrays (“wings”), illustratively a pair of wings 105-1, 105-2, extend in opposite directions from opposite sides or surfaces of main body 101. Similarly, differently sized, deployed reflectors/antennas 110, 111 extend in opposite directions from main body 101.
Typically, disturbance torques experienced by spacecraft 100 cause the spacecraft stored momentum to increase, which momentum is conventionally stored (i.e., absorbed) in a system comprising at least one spinning flywheel, variously referred to as a “reaction wheel” or “momentum wheel”, by changing the angular velocity of the at least one flywheel. Periodically, when the momentum storage capacity of the system is reached, momentum must be removed (“dumped”) by generating opposing momentum components, e.g., by firing thrusters of the spacecraft's reaction-control system. However, significant disadvantages associated with such approach for momentum control include the increased consumption of propellant, increased ground operator workload, and the need for accommodating the disturbance torques may affect the design of the spacecraft thruster configuration and redundancy.
By way of illustration, for MUOS the pitch gravity gradient with a single large (14 meter) reflector/antenna is ˜2,000 μin.-lb., which results in a pitch momentum accumulation of 172 in.-lb.-sec./day and the need for daily momentum adjust maneuvers.
In view of the foregoing, there exists a clear need for improved means and methodology for mitigating the above-described problems, drawbacks, and disadvantages associated with the conventional approaches for providing compensation of pitch gravity-gradient disturbance torque and other environmental torques experienced by spacecraft of the type contemplated herein.